1. Field of the Invention
The present invention relates to a head of a golf club and, more particularly, to an improvement in aerodynamic characteristics of the surface of a head of a golf club.
2. Description of the Related Art
Generally, the kinetic speed of a head of a golf club during the swing thereof increases during a down-swing of the club and is highest at the moment of impact with a golf ball. The maximum head speed attained by the average golfer during such a swing is usually in the range of from 30 to 50 m/sec, but to increase the fly distance of a golf ball, it is necessary to increase this maximum head speed. However, an aerodynamic drag, large enough to ensure that it cannot be neglected, is imposed on the head during the swing movement, and thus, to increase the maximum head speed and to stabilize the swing movement of the head, it is necessary to improve the aerodynamic characteristics of the surface of the head of the golf club.
When a conventional head of a golf club is swung, a laminar air flow boundary layer is produced on the outer surface of the head, and at the rear end of the laminar air flow boundary layer, the air stream is exfoliated from the outer surface of the head and a subatmospheric pressure region is formed behind the head. The air stream exfoliated from surface of the head is changed directly to a turbulent flow or swirling stream by the subatmospheric pressure.
In this air stream condition, air in front of the head is compressed and the air pressure increased, but behind the air stream-exfoliating point, the air pressure is reduced. Accordingly, an aerodynamic drag due to the pressure difference in front of and behind the head (hereinafter referred to as "pressure drag") is imposed on the head during the swing. It is known that such a pressure drag reduces the head speed during the swing and has an adverse affect on the stability of the movement of the head during the swing.
When a turbulent boundary layer in the air, i.e., an intermediate boundary layer at the point of transition from a laminar flow to a turbulent flow, is generated on the surface of the head, the air stream is not easily exfoliated from the head surface and the exfoliating point of the air stream is shifted to the rear part of the head. Accordingly, the subatmospheric pressure generated behind the exfoliating point of the air stream is reduced, resulting in reduction of the pressure drag acting on the head. When the relationship between the head speed during the swing and the pressure drag acting on the head is examined, it is seen that an increase of the head speed, causes an increase of the pressure drag, but at the point when the air stream boundary layer on the head surface changes from the laminar flow boundary layer to the turbulent boundary layer, the pressure drag is abruptly reduced and then, with the increase of the head speed, the pressure drag is also increased. The kinetic speed of a body at which the pressure drag is abruptly reduced is generally called the "critical speed", and this term is accordingly adopted in this description.
To increase the maximum head speed during the swing, preferably a turbulent boundary layer is formed on the head surface as soon as possible before the head speed reaches a highest level, thus reducing the pressure drag, and the degree of reduction of the pressure drag is increased. However, in case of a conventional head, the critical speed is higher than the maximum head speed attainable by an average player, and therefore, a large pressure drag is imposed on the head during the swing.
In a club head disclosed in Japanese Examined Patent Publication No. 53-31417, which corresponds to U.S. patent application No. 387,760, a groove, called a "trip step", is formed on the top edge of the face portion of a head body to produce a turbulent boundary layer in the air on the head surface. In a club head having such a trip step, a turbulent boundary layer can be generated in the stage where the head speed is relatively low, but the region where the turbulent boundary layer is formed is relatively narrow and, therefore, exfoliation of the air stream occurs at a position relatively close to the face portion of the head body. Accordingly, this club head is defective in that the degree of reduction of the pressure drag at the critical speed is small and the increase of the pressure drag with the subsequent increase of the head speed is large.